A. c. plasma display panel interrogating apparatus

ABSTRACT

A.C. plasma display panel interrogating apparatus including coincident addressing means for applying an interrogation signal to selected rows and columns of the panel cells in combination with the sustaining signal which is normally applied thereto such that the resultant signal across the coincident cell remains at the sustaining level while at the non-coincident cells the signal strength decreases below the extinction value. This condition reduces the non-coincident cell outputs to zero with only the coincident cell providing a signal output indicative of its data storage state, thereby enabling readout to be performed with a respective current or photo sensor arranged to receive the signals from individual rows or columns of cells.

May 9, 1972 United States Patent Schott [541 A. C. PLASMA DISPLAY PANELOTHER PUBLICATIONS INTERROGATING APPARATUS Proceedings-Fall JointComputer Conference, Bitzer et al.,

[72] Inventor: Dan J. Schott, Phoenix, Ariz.

[73] Assignee: Sperry Rand Corporation Primary Examiner-Malcolm A.Morrison Filed; May 25 1970 Assistant E.\aminer.lames F. Gottman AIt0rne\'S. C. Yeaton 40,238

[2|] Appl. No.:

[ ABSTRACT A.C. plasma display panel interrogating apparatus including340/166 EL coincident addressing means for applying an interrogation11/28 G1 1c 1 l/32- 7/00 signal to selected rows and columns of thepanel cells in com- F'eld of Search 173 PL bination with the sustainingsignal which is normally applied thereto such that the resultant signalacross the coincident cell remains at the sustaining level while at thenon-coincident cells the signal strength decreases below the extinctionvalue.

[52] US. Cl........ .......340/l73 PL, 3l5/l69 TV, 340/324 A,

[5]] Int Cl [58] .340/173 LT, 173 CR,

340/324 R, 324 A, 166 EL; 315/169 R, 169 TV References Cited UNITEDSTATES PATENTS This condition reduces the non-coincident cell outputs tozero H1971 with only the coincident cell providing a signal outputindica- 5/1970 3,559,190 Bitzer et al. 73 PL 3,513,327

tive of its data storage state, thereby enabling readout to be performedwith a respective current or photo sensor arranged to receive thesignals from individual rows or columns of cells.

14 Claims, 7 Drawing Figures Y SELECTION MATRIX 29 INTERROGATE SIGNALGENERATOR INTERROGATE COMMAND X SELFCTlON MATRIX P ATE NTEDHAY SHEEI 1[1F 3 SELECTION INTERROGATE SIGNAL GENERATOR INTERROGATE o COMMANDSELECTION MATRIX 11v VE/VTOR 041v J. 'SCHOTT A fro/m5 Y PATENTEDMY. 9I972 3,662,352

SHEU 2 BF 3 NON-INTERROGATING INTERROGATING MODE MODE 0 VS (9 VS"- E: 5E2 2; :2 20

0) VS 3 v DJ 2- VS E 8 2 2 m2 0: um i NJ?) I--- 2 "s E D Vs-- Vs-- wVOLTAGE o VOLTAGE E4 A 33 8d 1 do cuRRENT do w CURRENT m S'rs1- Q 8 Lu VVs... P o VOLTAGE 3 VOLTAGE AND 54 1 CURRENT) w" m4 w J v M 0 CURRENT 33 vs V 4 Vs.- I I I I o v 0 v 2 s VOLTAGE E s VOLTAGE gJ SJ {CURRENT EddV 535 V :10 CURRENT] V S v 3 Vs". z 2 b F I6 .20. FIG.

SELECTED I HORIZONTAL CELL ELEcTRooE VER'HCAL ELEcTRooE I Y SW INVENTOI?v w SELECT SELECT 8 DAN J. Sc /arr V SIN W? F lG.2c.-

ATTORNEY P'ATENTEDMAY 9 I972 ERROGATING CURRENT owkuwJww m zr awkowqwmzo VERTICAL ELECTRODE E) stw o m C m s 3 UL 0 n u m 2 HORIZONTALELECTRODE Y SELECT r W Y T0 E N N E n V5 m M 7 w A m N A. C. PLASMADISPLAY PANEL INTERROGATING APPARATUS BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention relates to A.C. plasmapanels and more particularly to means for interrogating or reading outthe information stored therein as represented by the instantaneousoperating condition of the individual cells comprising the panel.

2. Description of the Prior Art An A.C. plasma panel typically comprisesa two-dimensional array of gas discharge cells formed between two spacedparallel glass plates sealed together about their edges to form aunitary gastight structure confining an ionizable gas in the regionbetween the plates. A set of linear parallel electrode elements isformed on the exterior planar surface of each plate, the respective setsbeing orthogonally oriented relative to one another in a conventionalX-Y or row-column format. The individual cells of the array comprise thefinite gaseous regions located between the multiplicity of coincidentpoints formed by the crossed electrode configuration. Informationstorage is conveniently accomplished in accordance with conventionalcoincident addressing techniques simply by applying appropriate signalsto selected row and column electrodes so as to establish an ionizingpotential difference at the point of cioncidence of the selectedelectrodes. This produces a localized discharge in the gas at thecoincident point, indicative of data storage thereat. Thus, the panelcan be used for storing either binary data or symbols or other graphicaldata represented by correspondingly energized cell patterns.

In operation of the panel, an A.C. sustaining signal is connected to theelectrodes to establish a quiescent potential difference across eachcell at a level intermediate the extinction and ionizing potentials ofthe gas. Under this condition all the cells are initially off.Information can be written into selected cells by applying a write pulseto the corresponding pair of electrodes at the proper instant withrespect to the sustaining signal. The amplitude of the write pulse ismade sufficiently large so that in combination with the sustainingsignal it produces an instantaneous voltage level in excess of theionizing potential, thereby switching the selected cell to an oncondition. Ions and electrons flow to the glass plates of the selectedcell and accumulate thereon, rapidly providing a sufficientcounteracting electric field which extinguishes the discharge. Duringthe next half cycle of the sustaining signal, the field produced by theions and electrons briefly reinforces the applied sustaining signalfield causing another discharge to occur which also is rapidlyextinguished as explained above. Thereafter, with the applied potentialmaintained at the sustaining level, operation continues in the foregoingmanner with discharge current and light pulses being produced in eachhalf cycle of the sustaining signal. However, those cells for which theapplied signal amplitude was not momentarily raised above the ionizingpotential remain continuously off. A sustaining signal frequency on theorder of 50 kilohertz or higher, up to perhaps several hundredkilohertz, is used so that the light output appears continuous withoutperceptible flicker. Stored data is conveniently erased from the panelsimply by applying an erase pulse, similar to the write pulse, butphased differently with the sustaining signal.

Interrogation or readout of the stored data has customarily beenaccomplished heretofore by temporarily removing the sustaining signaland then selectively reapplying it to the individual cells. This actioncauses the individual cells to return to their original state, that is,cells that were off prior to removal of the sustaining signal remain offwhen it is reapplied whereas cells that were previously on switch onagain thereby producing current and light pulses indicative of the oncondition. Inasmuch as the readout is non-destructive, subsequentapplication of the sustaining signal simultaneously to all the cellsreproduces the panel image in preparation for further writing, erasingand interrogating. The length of time for which the sustaining signalcan be removed without deleteriously affecting the reproduced image is,of course, limited and in fact is determined by the life time of thedischarge produced charges on the interior walls of the glass plates. Inany event, the life time is generally sufficiently long to assurereadout without any difficulty.

SUMMARY OF THE INVENTION The present invention is particularly concernedwith improved means for reading out the data stored in an A.C. plasmapanel. Specifically, the invention provides for interrogating a panelwithout the necessity for switching off the sustaining signal. In oneembodiment this is accomplished by the provision of X-Y addressing meanswhich applies, to selected row and column arrangements .of the panelcells, interrogating signals that are equal in amplitude and frequencyto the sustaining signal but 180 phase shifted therefrom. As a resultthe cell located at the coincident point of theselected row and columnhas a net signal applied to it which is also equal in amplitude andfrequency and lphase shifted relative to the sustaining signal. Thecoincident cell therefore continues to function in the operational modewhich it had acquired prior to interrogation. At the non-coincidentcells, on the other hand, the interrogating signals cancel thesustaining signal so the resultant voltage is zero and the cells aretemporarily turned off. Respective current sensors connected in serieswith the cells of corresponding columns detect the current pulsesrepresentative of an on condition or data storage at the coincidentcell. Absence of such current pulses indicates an off condition or zerodata storage at the coincident point. Under appropriate control, forexample a computer, the interrogation process can continue with eachcolumn being addressed sequentially during concurrent addressing of asingle row and so on row by row in the manner of a conventionaltelevision raster scan. The computer, of course, maintains cognizance ofthe particular cell which happens to be the coincident one at eachinstant. It will be readily appreciated, therefore, that the A.C. plasmapanel which is normally used for displaying computer information canalso be used for conveniently feeding data into a computer under directcomputer control.

In another embodiment of the invention the interrogating signalscomprise one component having a frequency equal to that of thesustaining signal and another component at twice the sustainingfrequency. These signals are applied in the same manner to achieveessentially the same result as the aforedescribed embodiment, namely asustaining level signal at the coincident cell and a signal less thansustaining level at the non-coincident cells. In this case, however, thevoltage at the'non-coincident cells, although reduced below the levelrequired to sustain the discharge, does not diminish to zero. Inaddition, the output current or light pulses of the coincident cell,assuming prior data storage thereat, occur at twice the frequency of anyother cells which happen to be simultaneously providing output signals.This unique characteristic of the coincident cell output enables theinterrogation to be performed with a single sensor, either electricallyor optically. This characteristic also enables the interrogation to beperformed by simultaneously addressing all the cells in place ofcoincident addressing as will be explained more fully in the followingDescription of the Preferred Embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of aplasma panel interrogation apparatus constructed in accordance with theprinciples of the present invention.

FIGS. 2a and 2b depict waveforms illustrative of a double amplitudeinterrogation technique which may be employed in the apparatus of FIG.1.

FIG. 2c is a simplified schematic indicating the manner in which thedouble amplitude interrogate signals are applied to the selected cell.

FIGS. 3a and 3b depict waveforms illustrative of a double frequencyinterrogation technique which may be employed in the apparatus of FIG.1.

FIG. 3c is a simplified schematic indicating the manner in which thedouble frequency interrogate signals are applied to the selected cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, aplasma panel shown in exploded form comprises a multi-apertured shim 11disposed between plates 12 and 13, all of which are typicallyconstructed of glass or other suitable dielectric material. In assemblyof the panel the plates and shim are held in contiguous relation andsealed about their periphery to form a gastight structure for confiningan ionizable gas in the individual cells 14 formed in shim ll.Vertically extending electrodes 16, through 16, affixed to the exteriorsurface of plate 12 in superimposed relation with cells 14 are coupledthrough respective transformers 17, through 17, to X-select matrix 18.Similarly, horizontally oriented electrodes 19 through 19, affixed tothe exterior surface of plate 13 in superimposed relation to the cellsare coupled through transformers 21, through 21, to Y-select matrix 22.A sustaining level signal (V sin wt) is applied to all the cells fromA.C. source 23 which has one output terminal connected to ground and itsother output terminal connected by lead 26 to the secondary windings ofthe Y-select transformers, the A.C. signal path being completed throughthe individual gas cells to the secondary windings of the X-selecttransformers which, in turn, are connected by lead 27 back to thegrounded side of the A.C. source. Current sensors 28, through 28,,connected in series with the secondary windings of the respectiveX-select transformers, are included to detect the current pulsesgenerated within the respective cells for the purpose of interrogatingthe panel. This will be explained more fully in the subsequentparagraphs pertaining to the operation of the inventive apparatus.

Before proceeding with the description of the operation, however, itshould be understood that the inventive concept is not restricted to anyparticular constructional details insofar as the panel is concerned.Other cell arrangements and electrode configurations differing fromthose shown in the figure can also be used. For instance, a shim havinga single large centrally located aperture could be used in place of theillustrative multi-apertured cell which is shown simply to facilitatethe ensuing description.

Since the invention is concerned with means for interrogating the panel,it will be assumed that information has previously been written into thepanel as hereinbefore explained such that all of the cells, individuallydesignated by letters A through P, are in a lighted or on condition. Theinterrogation process commences in response to a computer or operatorinitiated interrogate command applied to interrogate signal generator 29which then applies signals to the appropriate electrodes by way of the Xand Y selection matrices. For example, during the time that a signal isapplied to transformer 21, to energize the top horizontal electrode,signals can be applied in succession to transformers 17, through 17, toscan or read out the top row of cells (A, B, C and D). Then thisprocedure can be repeated with transformers 21 21, and 21, successivelyreceiving excitation from the Y-select matrix. At the instant thatsignals are applied to horizontal electrode 19, and vertical electrode16,, cell A is the coincident (selected) cell while cells B, C and D andE, I and M are the non-coincident (half-selected) cells. All other cellsare designated as non-selected cells. Likewise at an instant whenelectrodes 16 and 19, are energized, cell F becomes the selected cellwhile cells E, G and H, and B, .I and N are half-selected and theremaining cells are non-selected.

Consider this latter condition in further detail. The selected cell F isthe one which it is desired to interrogate. As shown in FIG. 2a, whenthe interrogate signal is zero and only the sustaining signal V sin wtis applied to the panel, all the cells provide a current pulse outputduring each half cycle of the sustaining signal (for the previouslyassumed condition that all cells are in the on state). When aninterrogate signal is applied equal in frequency to the sustainingsignal but at twice the amplitude and out of phase therewith asindicated in FIG. 2b, the resultant signal across the selected cellbecomes V sin wt which again provides a current pulse during each halfcycle of the resultant signal. At the half-selected cells, however, thesustaining and interrogate signals cancel so that current pulses are nolonger generated in these cells. The nonselected cells, on the otherhand, still receive only the sustaining signal and therefore continue togenerate current pulses as they did prior to application of theinterrogate signal. The important consequence of the foregoing action isthat in the rows and columns including the selected and half-selectedcells only the selected cell provides output current pulses. Thus, therelated current sensor, in this case sensor 28,, detects informationfrom only selected cell F. In this way individual sensors associatedwith each row or column of cells are able to read out the informationfrom all the cells in that row or column, as the case may be. It will beunderstood, of course, that if the selected cell had not previously beendriven above the ionizing lever it would have remained dark continuouslyboth prior to and during interrogation. Since a light pulse is producedcoincident with each indicated current pulse, a photodetectorcooperating with appropriate light directing elements could be used inplace of each current sensor to perform the interrogation. As in thecase of the current sensor, only one photodetector would be required foreach row or column of cells. As a result of the aforementioned memorycharacteristic of the cells, at the conclusion of the interrogationperiod, when once again only the sustaining signal is applied to thepanel, all the cells return to the state in which they were at prior tointerrogation.

A specific circuit implementation for applying an interrogate signal ofthe foregoing nature is shown in FIG. 2c where the plus and minuslegends indicate instantaneous values of the A.C. signals. As indicated,the signals from the X and Y select matrices are both equal in amplitudeand poled opposite to the sustaining signal.

In another embodiment of the invention, the interrogate signal is of theform V (sin 2 wt sin wt), that is, it has one component equal inamplitude and frequency to the sustaining signal but 180 phase shiftedtherefrom and another component equal in amplitude but at twice thefrequency of the sustaining signal. This composite interrogate signal isdepicted in FIG. 3b. Prior to interrogation, the various waveformsappear as indicated in FIG. 3a which will be recognized as beingidentical to FIG. 2a. The interrogate signal can be applied to theselected electrodes in combination with the sustaining signal as shownin the simplified schematic of FIG. 3c. As in the previously describedembodiment, application of the interrogate signal causes cancellation ofthe sustaining signal at the selected cell and the production thereat ofa resultant signal equal to V sin 2 wt as shown in FIG. 3b. The selectedcell now provides a current pulse during each half cycle of theresultant double frequency signal, that is, at twice the frequency ofthe sustaining signal. In this case the voltage at the half-selectedcells does not reduce to zero but does decrease to a value less thanthat required to sustain ionization so that no current pulses aregenerated in these cells.

The maximum value of the resultant voltage at the halfseleeted cells canbe readily determined by differentiating the half-selected cell voltageV a (V sin 2 wt V sin wt) with respect to wt and setting the resultequal to zero. This analysis indicates a maximum value of about 0.88 Voccurring at 53 and 307 with smaller maxima occurring at 149 and 21 1 asshown in FIG. 3b. It is apparent therefore that the output of theselected cell can again be readily discriminated from the half-selectedcells. In all other respects, namely output of the non-selected cells,absence of output from non-ionized cells, and restoration of the displaysubsequent to interrogation, the operation of the double frequencyinterrogation technique is essentially similar to the double amplitudetechnique.

Another interesting advantage inherent in the double frequencyinterrogator relates to the output pulses of the selected cell occurringat twice the frequency of the sustaining signal. This permits theselected cell to be discriminated from not only the half-selected cellsbut also the non-selected cells. If current sensing is employed, anindividual sensor may be coupled to all rows or columns of cells;similarly, if photo sensing is employed, a single photodetector can beutilized.

Another unique feature of the double frequency technique is that itpermits readout to be performed without coincident addressing. This canbe accomplished, for example, by applying the interrogate signal incombination with the sustaining signal to a single row or column ofcells, say the row in superimposed relation with horizontal electrode 19The sustaining signal would be applied to all cells, as in thepreviously described cases. Then the row of cells receiving theinterrogate signal would provide current and light output pulses atdouble the frequency of the sustaining signal. Accordingly, a current orphoto sensor arranged to receive the signal from each column of cells,as in FIG. 1, would then be able to discriminate the interrogated celloutput signal from the output signals of the remaining non-interrogatedcells of each column.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appended claims without departing from the true scopeand spirit of the invention in its broader aspects.

I claim:

1. Apparatus for interrogating an A.C. plasma panel wherein informationstorage is represented by the ionization condition of each of aplurality of gas discharge cells, said apparatus comprising means forapplying to each cell a sustaining signal having an amplitude in therange between the extinction and ionizing levels whereby any cellactivated to a lighted state as a consequence of the cell voltagemomentarily exceeding the ionization level thereafter remains lightedunder the influence of the sustaining signal, and

means for applying an interrogate signal to prescribed cells of saidplurality of cells, the interrogate signal having one component whicheffectively cancels the sustaining signal in at least one of saidprescribed cells and an additional component which thereafter maintainsthe state of said one prescribed cell as it existed prior to applicationof the interrogate signal.

2. The apparatus of claim 1 wherein the additional component has anamplitude substantially the same as the sustaining signal and is furtherrelated thereto in one or two respects, one being a 180 phase shift fromthe sustaining signal and the other being a frequency which is aharmonic of the sustaining signal.

3. The apparatus of claim 1 wherein the panel includes respective setsof electrodes arranged transversely to one another on opposite sides ofthe cells and in superimposed relation therewith and further includingan interrogate signal source, and

means in said interrogate signal applying means for selectivelyconnecting said interrogate signal source to individual electrodes ofone set.

4. Apparatus for interrogating an A.C. plasma panel incorporating aplurality of gas discharge cells, comprising means for applying to eachcell a sustaining signal having an amplitude intermediate the extinctionand ionization levels of the cells, and

means for simultaneously applying first and second components of aninterrogate signal to first and second groups respectively of theplurality of cells, one cell being common to both groups and theinterrogate signal being operative to maintain the voltage at the commoncell above the extinction level while simultaneously reducing thevoltage across the other cells of said groups below the extinctionlevel. 5. The apparatus of claim 4 wherein the means for applying theinterrogate signals includes means for selectively applying theinterrogate signal components to different groups such that the commoncell can be selected as any one of the plurality of cells.

6. The apparatus of claim 4 further including means for sensing theoutput signal provided by said common cell in response to theinterrogate signal.

7. The apparatus of claim 4 wherein the interrogate signal is of thesame frequency and inverted in phase relative to the sustaining signal.

8. The apparatus of claim 4 wherein the one component of the interrogatesignal includes a frequency which is a first harmonic of the sustainingsignal frequency.

9. Apparatus for interrogating an A.C. plasma panel incorporating amatrix of gas discharge cells disposed between first and second sets oftransversely oriented electrodes having cross-over points defining thelocation of the respective cells, said apparatus comprising an A.C.signal source coupled to the first and second sets of electrodes toestablish a sustaining signal across each cell whereby any cellactivated to a lighted state as a consequence of the cell voltagemomentarily exceeding the ionization level thereafter remains lightedunder the influence of the sustaining signal, and

means for applying one component of an interrogate signal to a selectedelectrode of said first set of electrodes and applying an additionalcomponent of the interrogate signal to a selected electrode of saidsecond set of electrodes simultaneously with the sustaining signal suchthat the cell located at the cross-over of the selected electrodes has aresultant signal applied thereto exceeding the extinction level of thecell whereas at the remaining cells associated with the selectedelectrodes the resultant signal applied to the cells is less than theextinction level.

10. The apparatus of claim 9 wherein the interrogate signal is of thesame frequency but inverted in phase relative to the sustaining signaland approximately twice the amplitude thereof.

11. The apparatus of claim 9 wherein the interrogate signal includes onefrequency component equivalent to the sustaining signal frequency andother frequency component which is a first harmonic of the sustainingsignal frequency.

12. The apparatus of claim 9 further including means for sensing theoutput signal provided by the selected cross-over cell in response tothe interrogate signal.

13. The apparatus of claim 12 wherein each component of the interrogatesignal is of substantially the same amplitude and frequency as thesustaining signal but phase shifted therefrom.

14. The apparatus of claim 12 wherein each component of the interrogatesignal includes respective subcomponents of half the amplitude of thesustaining signal, one subcomponent having a frequency equal to butinverted in phase relative to the sustaining frequency and anothersubcomponent, having a frequency which is a first harmonic of thesustaining frequency.

1. Apparatus for interrogating an A.C. plasma panel wherein informationstorage is represented by the ionization condition of each of aplurality of gas discharge cells, said apparatus comprising means forapplying to each cell a sustaining signal having an amplitude in therange between the extinction and ionizing levels whereby any cellactivated to a lighted state as a consequence of the cell voltagemomentarily exceeding the ionization level thereafter remains lightedunder the influence of the sustaining signal, and means for applying aninterrogate signal to prescribed cells of said plurality of cells, theinterrogate signal having one component which effectively cancels thesustaining signal in at least one of said prescribed cells and anadditional component which thereafter maintains the state of said oneprescribed cell as it existed prior to application of the interrogatesignal.
 2. The apparatus of claim 1 wherein the additional component hasan amplitude substantially the same as the sustaining signal and isfurther related thereto in one or two respects, one being a 180* phaseshift from the sustaining signal and the other being a frequency whichis a harmonic of the sustaining signal.
 3. The apparatus of claim 1wherein the panel includes respective sets of electrodes arrangedtransversely to one another on opposite sides of the cells and insuperimposed relation therewith and further including an interrogatesignal source, and means in said interrogate signal applying means forselectively connecting said interrogate signal source to individualelectrodes of one set.
 4. Apparatus for interrogating an A.C. plasmapanel incorporating a plurality of gas discharge cells, comprising meansfor applying to each cell a sustaining signal having an amplitudeintermediate the extinction and ionization levels of the cells, andmeans for simultaneously applying first and second components of aninterrogate signal to first and second groups respectively of theplurality of cells, one cell being common to both groups and theinterrogate signal being operative to maintain the voltage at the commoncell above the extinction level while simultaneously reducing thevoltage across the other cells of said groups below the extinctionlevel.
 5. The apparatus of claim 4 wherein the means for applying theinterrogate signals includes means for selectively applying theinterrogate signal components to different groups such that the commoncell can be selected as any one of the plurality of cells.
 6. Theapparatus of claim 4 further including means for sensing the outputsignal provided by said common cell in response to the interrogatesignal.
 7. The apparatus of claim 4 wherein the interrogate signal is ofthe same frequency and inverted in phase relative to the sustainingsignal.
 8. The apparatus of claim 4 wherein the one component of theinterrogate signal includes a frequency which is a first harmonic of thesustaining signal frequency.
 9. Apparatus for interrogating an A.C.plasma panel incorporating a matrix of gas discharge cells disposedbetween first and second sets of transversely oriented electrodes havingcross-over points defining the location of the respective cells, saidapparatus comprising an A.C. signal source coupled to the first andsecond sets of electrodes to establish a sustaining signal across eachcell whereby any cell activated to a lighted stAte as a consequence ofthe cell voltage momentarily exceeding the ionization level thereafterremains lighted under the influence of the sustaining signal, and meansfor applying one component of an interrogate signal to a selectedelectrode of said first set of electrodes and applying an additionalcomponent of the interrogate signal to a selected electrode of saidsecond set of electrodes simultaneously with the sustaining signal suchthat the cell located at the cross-over of the selected electrodes has aresultant signal applied thereto exceeding the extinction level of thecell whereas at the remaining cells associated with the selectedelectrodes the resultant signal applied to the cells is less than theextinction level.
 10. The apparatus of claim 9 wherein the interrogatesignal is of the same frequency but inverted in phase relative to thesustaining signal and approximately twice the amplitude thereof.
 11. Theapparatus of claim 9 wherein the interrogate signal includes onefrequency component equivalent to the sustaining signal frequency andother frequency component which is a first harmonic of the sustainingsignal frequency.
 12. The apparatus of claim 9 further including meansfor sensing the output signal provided by the selected cross-over cellin response to the interrogate signal.
 13. The apparatus of claim 12wherein each component of the interrogate signal is of substantially thesame amplitude and frequency as the sustaining signal but phase shifted180* therefrom.
 14. The apparatus of claim 12 wherein each component ofthe interrogate signal includes respective subcomponents of half theamplitude of the sustaining signal, one subcomponent having a frequencyequal to but inverted in phase relative to the sustaining frequency andanother subcomponent, having a frequency which is a first harmonic ofthe sustaining frequency.